Charge control in electrostatic printing



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W, T. FISHER ET AL CHARGE CONTROL IN ELECTROSTATIC PRINTING Filed Sept.6, 1968 & W 8m my e IMT mfw h NITP I MDB e mrs M L25 a man i W w m 6 maC 0' mm 2 5 0% a, u G O I fir 3. /I Z MM //.P

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3,521,557 Patented July 21, 1970 3,521,557 CHARGE CONTROL INELECTROSTATIC PRINTING William T. Fisher, Los Alamitos, Robert D.Thompson,

Anaheim, Charles B. Patterson, Lakewood, and Stanley M. Dahl, Whittier,Calif., assignors to Purex Corporation, Ltd., Lakewood, Calif., acorporation of California Continuation-impart of application Ser. No.767,018, Aug. 26, 1968, which is a continuation-in-part of applicationSer. No. 651,946, July 7, 1967. This application Sept. 6, 1968, Ser. No.757,883

Int. Cl. B41m 1/12; B41f 15/00 US. Cl. 101114 12 Claims ABSTRACT OF THEDISCLOSURE REFERENCE TO COPENDING APPLICATIONS This application is acontinuation-in-part of our copending application Ser. No. 767,018 filedAug. 26, 1968, which is a continuation-in-part of application Ser. No.651,946 filed July 7, 1967, which is in turn a continuation of Ser. No.479,461, filed Aug. 13, 1965.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to improved methods and apparatus for printing, decorating or tolike purposes depositing printing particles such as fine electroscopicpigment by electrostatic means on surfaces of planar or curvilinearconfiguration, and is applicable to the direct labeling or decoration ofsheet or containers and other objects of irregular or intricate shapemade from paper,

plastic, glass, metal or combinations of these, or of other materialswhich have conductive or nonconductive surfaces. More particularly, theinvention is concerned with achieving more faithful replication ofcharacters, logos and like patterns of printed image by limiting freecharge accumulation at the image-defining stencil, to stabilize thestencil charge condition at desired levels for heightened fidelity ofimage to the stencil apertures.

Electroscopic toner particles, either solid or liquid, may be printed ona target surface 'by passage of the massed particles through a stencilwhich is positioned opposite and in closely spaced relation to thetarget surface. An electrical field is maintained between the target anda spaced voltage source within which field the stencil is positioned sothat electrically charged printing particles within the field areinduced to move through stencil aperture areas corresponding to thepictorial or literal matter to be printed, and to project onto thetarget surface.

Creation of the required electrical field is effected by the applicationof a voltage differential between a launch ing electrode and acounter-electrode. A stencil is placed between the electrodes as is thetarget surface for printing, so that stencil passing particles arecaused to deposit on the surface. Counter-electrode means may have anyof various particular forms, such as the target surface itself whereconductive, or a conductive backing for a non-conductive surface or aseparate electrode, in effect backing the target surface. In containerprinting, a counter-electrode inserted in the container, or projectedonly partially thereinto so as to ionize fluid within the container,creates an effective counter-electrode condition.

Prior art A continuing problem in electrostatic printing, attributableat least in part to sporadic free charge fiow to the stencil, is thedifiiculty of achieving perfect replication of the stencil aperture.Often the lettering is out of square, having attenuated corners andconcave intermediate portions, with an unfortunate randomness thatcauses a picturesque illegibility in the final print, particularly wherelettering is extensive and relatively small as, for example, useinstructions on a package.

In US. Pat. 3,364,853 issued to us on Jan. 23, 1968, we proposed amethod and apparatus for preventing free charge flow to the stencil. Thepresent invention is useful where it is not possible or convenient tocontrol fiow before it deposits on the stencil.

SUMMARY OF THE INVENTION The present invention is predicated on themajor concept, among others, of stabilizing the stencil charge conditionor potential relative to the target surface charge condition, and moreparticularly accumulating free charge flow to the stencil during andbetween successive printing particle launchings, away from the stencil.It has been found that at the high field strengths typically employed inelectrostatic printing for rapidity of particle movement, significant,if variable, amounts of free charge, resulting from movement from theelectrode of loosely bound charges not associated with the printingparticles, cascade from the launching electrode, traverse the gapbetween stencil and electrode and alter the charge condition of bothstencil and object, the former more than the latter, so that the stencilcharge alteration varies the relative potentials of stencil and object.Potential relationship is determinative of fidelity in printing tostencil opennigs. Nonuniform printing results, increasingly with time.Prevention of random charge increases on the stencil will limitvariations in field intensity and accordingly produce more uniform andfaithful printing. This prevention can be accomplished by accumulatingthe charges away from the field.

In particular, the invention provides a method of printing upon thesurface of an object that includes depositing electrically chargedparticles onto the surface through a spaced stencil and through anelectrostatic printing field maintained between the object surface andthe stencil in a manner permitting random free charge flows tending toalter in time the printing field intensity, and accumulating such chargefrom the stencil away from the printing field to thereby limit fieldintensity variation. In practice, the charge accumulation is carried outto maintain the charge condition of the stencil relative to the objectsurface at a constant value i.e., which does not deflect particlespassing therethrough and through the field be yond from their trajectorymore than is desired for final print appearance.

Charge accumulation is accomplished by a capacitance electricallyconnected to the stencil and away from the field, which capacitance actsto accumulate charge during charge flow to the stencil as needed tomaintain uniform printing field intensity. A resistance may be providedin the capacitance circuit to control the charge level of the stencilrelative to the object. In particular embodiments, the stencil may bepositioned within a field extending from the object surface to a voltagesource spaced from the stencil. Printing particles intended to bedeposited may be introduced preferably precharged into the field at thevoltage source side of the stencil. The method of the present inventionis particularly well suited to rapid, sequential printing of containersby passing electrically charged particles through the stencil and auniform field between the stencil and object.

Apparatus is provided for carrying out the above method of printing onthe surface of an object that includes means for forming an electricalfield including a voltage supply and stencil means within the field forselectively passing electrically charged particles to the surface, thevoltage supply including an electrode having a charge condition tendingto randomly subject the stencil to charge flows disruptive of thepotential relationship between the stencil and the object and meanslimiting accumulation of charge on the stencil, such as a capacitorelectrically connected to the stencil. A resistor in series or parallelwith the capacitor may be provided for control of charge levels on thestencil. In installation for the printing of containers, for which theinvention is particularly adapted, means are provided defining acontainer printing station within the printing field, said containerproviding the object surface to be printed.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of oneembodiment of the apparatus of the present invention;

FIGS. 2 and 3 are illustrations of printing characteristics that varywith field intensity; and

FIG. 4 is a graph of stencil and object voltage conditions with time.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring, now, to the drawingsin detail there is shown in FIG. 1 a typical improved printing apparatusaccording to the present invention, particularly adapted to print ordecorate an open-mouth container having a printable surface 10a.Printing means for depositing electrically charged particles upon thecontainer surface are shown including a printing particle supplycomprising a hopper 12 having a charging wire 14 connected to, e.g., apositive D.C. source (not shown) for charging printing particles 16,conveyor belt 18, suitably of rubber, cloth or mohair, and drivencontinuously beneath the hopper outlet between sheaves 19. Printingparticles 16, electrically charged within the hopper, fall by gravityonto the belt 18 and are carried to the launching area. There, alaunching electrode in the form of rotating cylinder 22 is providedconnected at 24 to, illustratively, the positive terminal 26 of a DC.voltage supply 28 so that the particles 16 are repelled by the electrode22 and are thus launched as a stream 20. A counter-electrode means isprovided to define an electrical field with electrode 22 includingconductive probe 32 positioned within container 10 at the open mouth orneck thereof. The probe is connected at 34 to, illustratively, thenegative terminal 38 of the DC. voltage supply 28, so that the particlesof stream are attracted to the counter-electrode and surface 10a. Withinthe field thus defined there is placed a printing pattern definingstencil 40 of dielectric material or preferably, conductive material,generally of brass or 'berylliurri-copper shim stock, which is typicallybiased to a desired charge value by connection to potentiometer 44.

The stencil 40 is provided with image defining apertures 40a whichpermit portions of printing particle stream 20 to pass the stencil andimpinge on the container surface. The stencil apertures may be producedmechanically as by blanking out with a suitable die or in a punch press,or chemically or electrochemically, by etching away unmasked areas ofthe sheet stock, or by other suitable means.

In the typical arrangement shown in FIG. 1, the electrical field willgenerally have a quite high potential, e.g., on the order of 20,000 tol00,000 volts or more. This is provided by application of a voltagedifferential 4 to the electrodes in a known manner and ordinarily byapplication of 40,000 to 60,000 volts to the launching electrode and10,000 to 20,000 volts of opposite polarity to the probe of thecounter-electrode.

For illustrative purposes herein, the launcher will be considered aspositively charged and the probe therefore negatively charged. Theparticles will then be positively charged. It is to be noted that it isthe relationship of polarities of the electrodes to one another and tothe particles and their absolute value that is of significance inprinting, not their sign. So, while the illustrative apparatus isdescribed herein with a positively charged launching electrode and anegatively charged counter-electrode and employing positively chargedparticles, all signs may be opposite.

Under the strong field conditions just described, there occurs aphenomenon known as free charge flow from the launching electrode to thestencil, as a result of the high field strength at the launcher drivingoff charge not asso ciated with the particles. This flow has been foundto have an irregular pulsing character, i.e. to be random or sporadic.Unless accumulated, as provided for in the present invention, the chargeflows to the stencil, even in the absence of electrical connectiontherebetween and independently of particle flow. The presence of thesecharges on the stencil alters the stencil charge condition which hasbeen selected to be at a particular value relative to the object priorto commencing operations and which is desirably stabilized at theselected relative level, thus to insure uniform or constant fieldintensity during printing. The potential between stencil and object isthus altered and printing field changed. Moreover these charges may tendto move to the aperture perimeters and there produce local chargeconcentrations which may significantly affect direction of particlemovement beyond the stencil to the object. While it is contemplated thatthe stencil 40 during printing may have any desired closeness ofapproach to the container surface 10a, generally it is desirable tomaintain at 36 sufiicient spacing to preclude contact smearing of thedeposited particles. The spacing at 45 between the launching electrodeand stencil, however, is relatively wide, typically in the range of 3 to15 centimeters, to promote and insure dispersion of particlesconstituting the stream 20 for free passage through the stencilopenings.

Generally, the stencil charge condition is initially set at a 'valuesuch that the ratio E /E is maintained between about 0.6 and about 3.0where V being the voltage of the launching electrode 22, V the voltageof the counter-electrode or counter-conductive means 32, V the voltageof the stencil 40, L is the distance (in centimeters) between thelaunching and counter-electrode means, and L, is the distance betweenthe stencil openings and counter-conductive means. Where the printingtarget surface serves as the counterelectrode, or has a voltagecorresponding substantially to that of an associated counter-electrode,then in the above equations V may be considered to be the voltage atthat surface, and L, and L are measured in relation to the surface. Whenthe counter-electrode does not conform to the target surface voltage,the field voltage at the surface is determined and used for V 1 beingthe distance between the stencil apertures and the target surface.

Typically use is made of a launching field strength of about 6,000 v./cm. to about 20,000 v./ cm. Essentially the same image may be presentedwhen both the launching field strength (E and air gap field strength(E,,) are increased by the same factor to maintain a constant ratio E /EFactors in the selection of field strength are the configuration of theobject to be printed, the spacing between the launcher and object andthe printing particle characteristics. It has been found that image ofgood clarity may be produced using an air gap spacing L less than about/4 inch. However, it is preferable to keep this spacing at around 0.07inch or less when printing fine detail. Generally, the closer thestencil openings are to the target surface, the more sharply defined theimage.

When the ratio E /E is too low, as, in an illustrative situation,substantially below 1.0, the resulting image of a large block letter maytend to be distorted with bulging sides, indistinct edge definition,acute inside corners and rounded outside corners, on the other hand, anincrease in the E /E ratio to greater than about 1.8 may produce anarrowing effect with sharply pointed corners as in FIG. 3. With theratio maintained between about 1.0 and 1.8 the image is of gooddefinition in keeping with FIG. 2. The potentiometer 44 provides avariable control to maintain the desired E /E, ratio, gene-rally bymaintaining the charge on the stencil at a level closely approximatingthe local field strength.

Obviously, in view of the foregoing ratio analysis, the image fidelityfactor is influenced by the stencil charge condition. Variations fromthe preset level, i.e. increases, due to relatively great free chargeflows onto the stencil relative to the object can change the printedimages in the manner illustrated in comparing FIGS. 2 and 3. Thisinvention is concerned with accumulating free charges flowing to thestencil away from the stencil thereby to stabilize the printing fieldbetween stencil and object at a predetermined constant level.

Referring again to FIG. 1, the surface 22a of the electrode 22 receivesby gravity feed the charged particles 16 from the belt 18 and conveysthem angularly to opposite the stencil 40 and thus into the electricalfield between electrodes 22 and 32. Once in the field, the particles,which it will be remembered are charged to the same sign as theelectrode 22, are repelled. Assuming a launching level field, as abovedescribed, the particles so leave the surface of launcher 22 and areelectrically attracted to the target surface a. Independent of particleflow, free charges on the launching electrode 22 will leave theelectrode and travel toward an adjacent attracting potential e.g. thecounter-electrode or the stencil beween the electrodes. It has beenfound that such free charge flow occurs continually, although not atconstant levels. It is with the minimizing of the deleterious effect ofsurges of free charge flow that the present invention is concerned. Withreference to FIG. 4 the effects of free charge flow in an apparatus likethat depicted in FIG. 1 is shown graphically. The voltage (chargecondition) of the printable surface and that of the stencil bothincrease 'with time. This is shown by a comparison of lines A (objectvoltage or charge condition) and B (stencil voltage or chargecondition). This increase is attributable to the continuing fiow of freecharge from the launching electrode 22. The difference between lines Aand B represents the voltage difference between stencil 40 and objectsurface 10a, i.e. the printing field intensity. It will be noted thatlines A and B are parallel and that the field intensity is uniformduring the time period shown in FIG. 4. With such a uniform fieldcondition printing results are sharp because of the absence ofdis-focusing effects of field intensity variation. The potential of thestencil 40 tends to increase, however, beyond the increase in potentialof the object surface 10a if the free charges remain on the stencil.

Such free charges remaining on the stencil are sufficiently proximatethe printing field between stencil 40 and object surface 10a to affectthat field. If this field is too high in intensity the printed imagesare attenuated (FIG. 3), if too low the images are bloated inappearance. If the field fluctuates during printing, due for example toa variation in the stencil potential a combination of image effectsresults, generally giving a fuzzy appearance. In FIG. 4 the rise instencil voltage from free charge thereon is shown on dotted line C. Itwill be seen that the field intensity between the stencil and objectchanges continually with time. Uniform field intensity conditions of theproper value i.e. appropriate relative potentials of object and stencil(see discussion above) produce perfectly formed letters like that shownin FIG. 2.

From the foregoing it is apparent that for successful printing freecharge fiow surges must be prevented from altering the field intensityand that therefore charge must be prevented from accumulating on thestencil. This invention provides for accumulation of the free chargesaway from the stencil, and thus the field, by accumulating thesecharges, i.e. absorbing them, in an electrical sense, in a capacitor.

In FIG. 1, the stencil 40 is shown electrically connected through line46a to capacitor 46 which is connected to potentiometer 44 at point ofneutral voltage. By this arrangement surges in the voltage level of thestencil 40 from free charge cascades by the launching electrode 22 arewithdrawn from the stencil and stored in the capacitor 46. The relativevalue of the stencil potential, to the object, initially is controlledby the applied potential of the potentiometer. The resistor 50 may becombined with the capacitor 46 in either parallel or series, preferablythe former as shown, with both elements performing their respectivefunctions, the resistor determining the voltage drop and the capacitorlimiting sharp changes in stencil potential from flow of charge to thestencil.

Example 1.Electrostatic printing was effected on a polyethylenecontainer with accumulation of free charge flow by use of an apparatussimilar to that shown in FIG. 1 using Sun Chemical Electrosc'opic Ink#CR 5559-40 having an average particle size of about eight microns. Thelaunching electrode was charged to 40,000 volts positive and the probecounter-electrode to 8,000 volts negative. The stencil was connected tothe potentiometer 28 through a 0.16 microfarad capacitor. No resistorwas used. There was free charge flow from the launching electrode. Auniform field -was maintained between the stencil and the containersurface throughout printing and literal information was sharply definedand undistorted.

Example 2.The apparatus of Example 1 was altered by substituting a 0.10microfarad capacitor for the 0.16 capacitor used there. Printing waseffected. Results were equivalent.

Examples 3 and 4.Example 1 was duplicated with a resistor connected inparallel with the capacitor. In Example 3 a megohm resistor was used. InExample 4 a megohm resistor was used. In both instances sharply definedand faithful reproduction of stencil openings was achieved on the bottlesurface, through maintenance of a uniform printing field.

As the printing particle material, a uniformly fine powder is preferred.A suitable toner powder has a particle size of about 5 to 10 microns. Alow temperature melting resinous powder may be used, if it is desired tofix the toner particles by heat, desirably not so low that the particlessinter in storage. A satisfactory power is essentially composed of anon-tacky, low melting and suitably colored natural or syntheticresinous material. The composition of the toner resin does not appear tobe critical insofar as electroscopic launching ability and imageformation are concerned. A number of suitable electroscopic resins aredescribed in US. Pat. No. 3,079,- 342. It is also possible in theinstant invention, to use an electroconductive powder consisting offlaked and polished aluminum powder having a particle size of less thanabout 40 microns in diameter. The aluminum powder readily produces animage under the same conditions as employed with resin-based pigmentparticles, the resulting image, however, must be subsequently fixed tothe surface as by application of a clear coating material. A finedispersion of a liquid toner can be used in lieu of the pigment powder.When reference is made to a pigment powder herein, it will be understoodto include not only materials that may be pigments or dyes themselves,

but also resins and other materials to which pigments or dyes are addedor which are otherwise given color including white and black. Techniquesfor fixing or otherwise subsequently treating the printed images arewell known in the art.

To summarize the operation of the invention, printing particles 16 areelectrically charged, are launched in a stream 20 by a like-chargedlaunching electrode 22 and pass through stencil 40' and impinge onselected areas of a target such as container 10 behind which acounterelectrode condition is maintained by a charge on probe 32. Thefree charge flow normally incident to the highly charged condition ofthe launching electrode is accumulated after reaching the stencil 40 byprovision of a capactor 46, optionally in combination with a resistor 50to accumulate cascades of free charge flow away from the field betweenstencil '40 and counter-electrode 32. With free charge accumulation thuscontrolled, the stencil charge condition, typically biased by adjustmentof potentiometer 44 to approximate the local field strength, isstabilized despite free charges flowing from the launching electrodewhich, unless accumulated would remain on the stencil to upset itscharge condition and disrupt printing field uniform intensity. In thisevent, erratic field strengths variably alter the trajectory ofparticles passing the stencil and cause oddly formed images. Byaccumulation of free charge flow in accordance with this inventionprinting uniformity is enhanced.

We claim:

1. The method of printing upon the surface of an object that includeslaunching electrically charged particles from an electrode toward saidsurface and through an image defining stencil there-between and throughan electrostatic printing field maintained between said surface and saidstencil in a manner subjecting the stencil and object simultaneously todifferent increases in charge thereby tending to change the printingfield intensity in time, and passing charge from the stencil toproportion increases in the stencil potential relative to increases inthe object surface potential to substantially eliminate variations inthe intensity of the printing field.

2. Method according to claim 1 including accumulating charge from thestencil in a capacitance electrically connected to said stencil tocontrol charge accumulation on the stencil.

3. Method according to claim 2 including biasing the stencil to apredetermined potential.

4. Method according to claim 2 including also main taining a secondfield between said object surface and the launching electrode andintroducing said particles into said second field at the launchingelectrode side of said stencil.

5. Method according to claim 1 including limiting the increase inpotential of the stencil to the increase in object surface potentialduring printing to maintain constant the field intensity therebetween.

6. Method according to claim 1 in which said object is a container andincluding also positioning said container at a printing station andpassing said electrically charged particles through said stencil ontosaid container surface.

7. Method according to claim 6 including providing a capacitanceelectrically connected to said stencil to control charge accumulation onthe stencil.

8. Apparatus for printing upon the surface of an object that includesmeans for forming an electrical field including a voltage supply, andstencil means within said field for selectively passing electricallycharged particles to said surface, said voltage supply including anelectrode subjecting the stencil and the object surface simultaneouslyto different increases in charge thereby tending to alter the potentialrelationship between the stencil and the object surface over time andmeans collecting charge from said stencil including a capacitorelectrically connected to the stencil to substantially eliminatevariations in the potential between the stencil and said object.

9. Apparatus according to claim 8 including also a resistor biasing saidstencil.

10. Apparatus according to claim 9 in which said resistor and capacitorare connected in parallel.

11. Apparatus according to claim 8 for printing a container includingalso means defining a container printing station within said electricalfield, said container defining said object surface.

12. Apparatus according to claim 11 including also a counter-electrodeinserted within said container.

References Cited UNITED STATES PATENTS 3,301,179 1/1967 Johnson 10lll43,364,853 1/1968 Fisher et a1. l0l114 EDGAR S. BURR, Primary ExaminerU.S. Cl. X.R.

